In the context of different projects both in the fields of communication and navigation, receivers with adaptive antenna arrays are developed which are adapted to orientate a plurality of independent antenna beams to different transmitters, e.g. to satellites, and to simultaneously suppress interference signals. For testing and designing such receivers, signal simulators are required that allow to generate reproducible signals that can be supplied to the receiver in the laboratory.
One of the main sources of interferences in satellite navigation are multipath signals, i.e. signals that impinge on the antenna of the navigation receiver only after having been reflected once or several times, e.g. by buildings. For satellite navigation, for example, receivers exist that are adapted to generate signals from a plurality of GPS satellites and, in the future, also Galileo satellites. However, these receivers only have relatively limited possibilities to factor multipath propagation in; the output signals include no information on directions.
In order to test an antenna-receiver unit, in particular the adaptive orientation of the antenna beams and the suppressing of interference signals, however, also the direction of incidence of the satellite signals has to be simulated. This can either be done using distributed transmitter antennas in a large anechoic measuring chamber or by means of an additional device, in which the phase shifts between the individual antenna elements are simulated according to the direction of incidence of the satellite signals, i.e. in which a plane wave front is generated for each input signal, which is incident from the direction of the associated satellite. The receiving antenna itself is simulated as well and the simulated signals are fed directly into the receiver input.
Existing signal simulators, e.g. the Spirent STR 4790 (STR 4790 Multi-Output GPS Signal Simulator Product Specification, MS3010, Spirent Communications), are able to generate up to twelve individual GPS satellite signals on two carrier frequencies. Hitherto, these simulators have been used in two ways to test receivers with antenna arrays having adaptively controllable receiving characteristics.
In the first way, in a large anechoic measuring chamber, the RF outputs of the simulators were connected to individual transmitter antennas spatially distributed in the measuring chamber. The receiving unit, i.e. the receiver including the antenna array, was placed on a motion platform. The movement of the satellites was simply simulated by rotating the motion platform (N.J. Boasman, P. Briggs: “The Development of an Anechoic GPS Test Facility”, Paper presented on ION National Conference, Albuquerque, N. Mex., 24.-26. Jun., 2002). A simulation using distributed antennas in an anechoic measuring chamber is disadvantageous in that, on the one hand, a rather large measuring chamber is required for far field measuring, which may have dimensions of several meters in all directions, and that, on the other hand, the movement of a satellite can only be simulated imperfectly.
In the second way, the RF outputs from the simulator were supplied to an analog device in which the plane wave fronts were generated using analog phase shifters and attenuating elements. The movements of the satellite and of the user were simulated by readjusting the phase shifters and the attenuating elements. Thus, up to eight antenna elements could be simulated at the output with sixteen input signals (D. Howell, D. Jacobs, B. Rahn, G. Green: “Virtual Flight Testing—A Versatile Approach to Evaluate Future GPS Anti-Jam Technologies”, Proceedings of ION NTM 26.-28. Jan. 2000, Anaheim, Calif., USA).
The disadvantage of an analog device for generating the plane wavefronts consists in the high hardware effort for the realization, which increases proportional to the product of the input and output signals. Compared to a digital solution, the analog variant is much less versatile, since modifications always require a new hardware design. An additional multipath channel simulation within this analog unit is not possible.
From A. Hornbostel, A. Schroth, H. Denks, M. Holbrow: “A New Signal Simulation Tool for Testing of Receivers with Controlled Reception Pattern Antennas”, ION 2004, Long Beach, 21.-24. Sep., 2004, and from A. Hombostel, A. Schroth, H. Denks, A. Konovaltsev, H. Venus, M. Holbrow: “A New Approach for Testing of Receivers with Controlled Reception Pattern Antennas”, Proceedings of Navitec 2004, ESA/ESTEC, Nordwijk, 8.-10. Dec., 2004, a digitally operating signal simulation tool for testing and developing GNSS receivers, especially for GPS and the future Galileo, is known which are adapted to orient a plurality of independent antenna beams to different signal-emitting radio transmitters and to simultaneously suppress interference signals and which comprise an adaptively controllable antenna array assembled from elements. This known simulation tool, which is also based on the above mentioned signal simulator “Spirent STR 4790”, includes a signal generator generating a plurality of digital signals in the complex baseband and a matrix processor.
The input signals of this matrix processor are the digital signals generated in the complex baseband by the signal generator and its output signals form the received signals of the individual elements of the antenna array of the radio receiver. Within the matrix processor, the input signals of the matrix processor are modulated in the digital portion by phase shifting corresponding to their direction of incidence and to the relative positions of the elements within the antenna array of the radio receiver. The phase-shifted signals are then combined and supplied to the individual outputs of the digital matrix processor. The main feature of this simulation tool is the generation of individual digital baseband satellite signals that, using an external digital matrix processor, can be converted to the input signals of the individual elements of an antenna array, the matrix processor performing a wave front generation for each satellite signal, which depends on the direction of incidence, and delivering the combined baseband signals for each antenna element as the output signal.
The output signals from the matrix processor may either be fed directly into a baseband receiver or they can be returned to the signal generator for conversion to RF signals. This simulation tool already allows for the testing and developing of satellite navigation receivers with an adaptively controllable antenna array for orienting a plurality of independent antenna beams in a small laboratory and requires no large anechoic measuring chamber.